[R] Plotting a cubic line from a multiple regression

[Stephan Kolassa]

Hi Ashish,

I am rather more concerned about whether what you outlined is legitimate 
(your question 1 below). If you are looking at children, higher AGE will 
be associated with higher TIV, so both variables would essentially 
measure the same thing (see Miller & Chapman, Misunderstanding Analysis 
of Covariance), which I really would not like to see.

Of course, if you are looking at adults, this will probably not apply.

Best wishes,
Stephan


Ashish Ranpura schrieb:
> 
> Dear all,
> 
> I'm attempting to plot a cubic relationship between two variables 
> controlling for the effects of a third variable. In this short example, 
> I'm trying to use AGE to predict CORTEX while controlling for the 
> effects of TIV (total intracranial volume):

(snip)

> Although the last 'lines' command fails, summary(cortex.lm) does give me 
> reasonable coefficients for the multiple regression. Can anyone advise 
> me if (1) this is at all a legitimate procedure, and (2) how to draw the 
> cubic function that fits the multiple regression?